The semiconductor industry continues to develop lithographic technologies which are able to print ever-smaller integrated circuit dimensions. Extreme ultraviolet (“EUV”) light (also sometimes referred to as soft x-rays) is generally defined to be electromagnetic radiation having wavelengths of between 10 and 120 nanometers (nm). EUV lithography is currently generally considered to include EUV light at wavelengths in the range of 10-14 nm, and is used to produce extremely small features, for example, sub-32 nm features, in substrates such as silicon wafers. To be commercially useful, it is desirable that these systems be highly reliable and provide cost effective throughput and reasonable process latitude.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has one or more elements, e.g., xenon, lithium, tin, indium, antimony, tellurium, aluminum, etc., with one or more emission line(s) in the EUV range. In one such method, often termed laser produced plasma (“LPP”), the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the desired line-emitting element, with a laser beam at an irradiation site. The line-emitting element may be in pure form or alloy form, for example, an alloy that is a liquid at desired temperatures, or may be mixed or dispersed with another material such as a liquid.
In some prior art LPP systems, droplets in a droplet stream are irradiated by a separate laser pulse to form a plasma from each droplet. Alternatively, some prior art systems have been disclosed in which each droplet is sequentially illuminated by more than one light pulse. In some cases, each droplet may be exposed to a so-called “pre-pulse” to heat, expand, gasify, vaporize, and/or ionize the target material and/or generate a weak plasma, followed by a so-called “main pulse” to generate a strong plasma and convert most or all of the pre-pulse affected material into plasma and thereby produce an EUV light emission. It will be appreciated that more than one pre-pulse may be used and more than one main pulse may be used, and that the functions of the pre-pulse and main pulse may overlap to some extent.
Since EUV output power in an LPP system generally scales with the drive laser power that irradiates the target material, in some cases it may also be considered desirable to employ an arrangement including a relatively low-power oscillator, or “seed laser,” and one or more amplifiers to amplify the pulses from the seed laser. The use of a large amplifier allows for the use of a low power, stable seed laser while still providing the relatively high power pulses used in the LPP process.
Systems currently known and used in the art typically set a fixed pulse width for the main pulse that is expected to produce the greatest amount of EUV energy under ideal conditions. The drive laser RF pump power that is applied to the amplifier is then adjusted through an RF generator, which uses pulse width modulation (PWM) to adjust the duty cycle (the fraction of the operating time that RF power is generated) to obtain the maximum or desired amount of EUV energy.
This approach has several limitations. First, it is relatively slow in comparison to the operation of the system. The laser power can only be changed in small amounts from one pulse to the next, and thus when the duty cycle is changed the system typically takes a number of pulses to change output.
In addition, the main pulse and pre-pulse typically go through the same amplifiers, since providing a separate set of amplifiers for each pulse is generally considered to be prohibitively expensive. If both pulses are amplified by the same amplifier, any change in the gain achieved by changing the duty cycle affects both pulses. However, it is common for the pre-pulse to be optimized to produce desired effects on both the expansion and trajectory of the target droplets; thus, a change in the duty cycle which also changes the energy in the pre-pulse might have detrimental effects on the performance of the system. It would thus be preferable for targeting stability to maintain a constant pre-pulse energy and adjust only the main pulse if possible.
Accordingly, it would be desirable to be able to adjust the EUV output energy in such an EUV light source faster than by adjusting the duty cycle and by adjusting the duty cycle as little as possible, and also to be able to adjust the energy of the main pulses without also adjusting the energy of the pre-pulses.